Series resonant inverters, including series resonant inverters which are parallel loaded, are shown for example in the GE SCR Manual, 5th Edition, 1972, Section 13.2.1.1, page 354. Page 354 shows a Class A series resonant half bridge inverter which uses a Silicon Controlled Rectifier (SCR) as the electronic switch. The same reference, at page 390 (Section 13.3.2.4) describes current limiting. Needless to say, however, the circuits already referred to are capable of significant improvement.
One particular problem which was reflected in many prior art inverter circuits relates to the manufacturability of the circuit. More particularly, and especially with series resonant inverters, some control is needed to ensure that the two switches making up a half bridge conduct alternately and there is no period of time during which the switches conduct simultaneously. One technique described in Buenzli U.S. Pat. No. 4,042,855; Nilssen U.S. Pat. No. 4,184,128 and Wellford U.S. Pat. No. 3,248,640 is to use one or two saturable direct drive inductors with resonant current driven primaries to control the input drive of the switching elements. The inductors saturate during normal inverter operation to control the operating frequency and enhance the vacation of minority based carriers during the turn off transitions of the switching elements. This action improves the turn off speed and reduces excessive device dissipation. While this technique is effective, it tends to be difficult to implement in production because of the tight relationship between the inductor saturation time and the inverter's resonant frequency, especially when one takes into account the need to use components with reasonable tolerances. In order to operate properly (and not destructively) the inductors must saturate either slightly before the inverter's resonance period or exactly on the inverter's resonance. If allowed to saturate over a longer time than the resonance, the inverter will self-destruct due to conduction overlap of the switching devices. If too short a saturation time is used, inverter efficiency suffers. In normal production of such a design, normal variations in switching device parameters require "tweaking" or trimming of the resonant inductor and/or the turns on the saturable drive inductor.
The Wellford and Buenzli patents use a single saturating inductor connected directly across the bases of a push-pull inverter to evacuate the base minority carriers and control frequency. However, base drive is supplied by a resistor connected common secondary of the main inverter transformer and thus the common secondary conducts on both half cycles of the inverter. Nilssen describes a separate saturable inductor using primary windings which conduct on both half cycles of inverter operation.
Another problem with prior art power conditioning electronics for driving fluorescent lamps is the power factor presented at the power input terminals. It should be apparent that as the power factor can be increased towards unity, more effective use of the input power is exhibited. Perper, in U.S. Pat. No. 4,017,784 and Knoll, in U.S. Pat. No. 4,109,307, describe using inverter feedback to achieve improved input current crest factor and power factor. Both designs use a feedback voltage and current which is in parallel with the normal load current to charge a storage capacitor which serves to supply energy to the inverter during times when the input voltage is below the stored voltage level. When the input voltage is above the stored voltage level, the inverter receives energy from the AC input signal and the storage capacitors are recharged. This technique, although improving power factor, carries a penalty in that since the load current and capacitor charging current are summed at the inverter output, additional power loss is realized in the switching devices as compared to an inverter operating without this feature.
The characteristics of typical inverters exhibited under noload conditions is such as to require some technique for providing shock protection. See Nilssen U.S. Pat. Nos. 4,461,980 and 4,663,571. The first-mentioned patent describes an inverter disabling means to protect the series resonant inverter from self-destruction due to high peak currents under no-load conditions which would exist if a lamp were removed during normal operation. Notwithstanding this technique, however, a problem of safety may arise due to the fact that the inverter power supply lines are still connected to the inverter circuitry after any shut down is effected. Shut down is accomplished in the prior art by disabling the switching devices through various means while leaving the AC power lines energized.
Another perennial problem in power conditioning electronics for driving (so-called) rapid start fluorescent lamps is the issue of filament current, and moreover, the entire starting operation. In this type of fluorescent lamp, filament current is needed for starting purposes, in order to initiate ionization or arcing. However, once a lamp is started, there is no need for filament current. Many prior art power conditioning devices provide a switch (timed or tied to inverter voltage) to terminate filament current at an appropriate time. See for example Kohler U.S. Pat. No. 4,375,608; Josephson U.S. Pat. No. 4,388,562; Bay U.S. Pat. No. 4,396,866 and Nilssen U.S. Pat. Nos. 4,581,562 and 4,652,797. Over and above the issue of filament current control, however, is the more pressing distinction between the turn on phase of typical fluorescent lamps and incandescent lamps. Energizing an incandescent lamp produces a sharp, clean and pleasing transition in which light is almost instantaneously available. This contrasts sharply with the starting phase of many fluorescent lamps which is first delayed from the time the lamp is energized (the switch is thrown by the user) and then starting occurs with one or more flickers of light. It would be an advantage to provide a method for energizing a fluorescent lamp which exhibited the unenergized/energized transition which is identical to or more nearly like that exhibited by incandescent lamps.